FIG. 6 is a sectional view of a conventional ultrasonic transducer configuration (prior art). In the figure, number 1 indicates an ultrasonic oscillator, 2 indicates an acoustic impedance matching member installed in front, 3 indicates an acoustic damper, and 4 indicates the subject.
Conventionally, a member having a thickness equivalent to a single or three-layer quarter-wavelength plate is used for acoustic impedance matching layer 2. The energy transmission efficiency and bandwidth are both improved when the number of quarter wavelength plates and the acoustic impedance of the material are properly selected. It is also known, however, that an acoustic impedance matching layer consisting of n layer of a quarter-wavelength plate fails to obtain a major additive effect as compared to when n=2 if n is made n&gt;or=3.
Therefore, an improved version of acoustic impedance matching layer 2 is proposed (as noted in published Patent No. Showa 58-18095) that successively changes the acoustic impedance between the respective layers in contact with ultrasonic oscillator 1 and the surface of the subject. However, it is not easy to actually. produce a composite material having a gradually changing acoustic impedance, or if produced, a lack of reproducibility poses a problem.